Apparatus and process for operating a dry-compression vacuum pump

ABSTRACT

A process for operating a dry-compression vacuum pump with at least one pump chamber and at least one rotor driven by an electric motor, and to a vacuum pump suitable for this operating process. To reduce operational shutdowns for maintenance purposes without any risk to the pump, it is proposed that application-specific phenomena leading to deviations from the normal operating status or their effects be monitored and, after a predetermined deviation quantity has been attained at which operation of the vacuum pump could be adversely affected in future, steps be taken to remove the causes thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for operating a dry-compressionvacuum pump.

2. Discussion of the Related Art

Dry-compression vacuum pumps are pumps which have oil-free pumpchambers. The advantage offered by these pumps is that they are capableof generating a vacuum which is free of hydrocarbons. Therefore, theyare preferably employed for the evacuation of chambers in which etching,coating or other vacuum treatment or vacuum production processes arerun. In particular Roots pumps or multi-stage claws pumps arewell-proven in practice as dry-compression pumps, i.e., dual shaftvacuum pumps having one or several stages. A pair of rotors is locatedwithin each stage. The rotating motion with respect to the rotorsthemselves and with respect to the walls of the pump chamber is suchthat no contact is established.

When employing dry-compression pumps in connection with the processesmentioned above, or also for evacuation of vacuum chambers in whichchemical processes are run, it is often the case that--depending on thetype of application--solids or liquids enter the pump. As long as theseare only pumped through the pump, they will hardly impair operation ofthe pump. However, generally these substances are chemically aggressiveor at least chemically or physically reactive, so that these may causeabrasions (in the case of liquids, for example) or formation of layers(in particular, when dust is in involved). In both cases there existsthe danger of reduced service life for the vacuum pump. In the presenceof aggressive liquids, wear effects are increased. The danger of dustexists in particular when the pumping semiconductor process gases. Inthis kind of application the circumstances may be such that extremelyfine solid particles may be formed during the compression phase of thegases, i.e. when the gases which are to be removed pass through thevacuum pump. Solid particles which enter the vacuum pump either directlyor indirectly may deposit themselves in the pump chamber, where they atfirst only cause a narrowing of the slit between the rotors. Furtherdeposits may cause the rotors to touch, so that the solid particles arerolled on to the surfaces of the rotors. When the deposits increasefurther, then the layer which is rolled on to the rotors thickens, sothat a force is created which forces the rotors and thus the shafts ofthe rotors apart. In particular, when the rolled on layer increasesfurther, this will damage the bearings and result in a failure of thepump.

For the reasons described, dry-compression vacuum pumps which areemployed in connection with the applications described, are, for safetyreasons, subjected to frequent maintenance, and this not only with theaim of preventing a reduction of service life, but also to preventoperational shutdowns due to malfunctions in the operation of the vacuumpump. However, operational shutdowns will also be required for necessarymaintenance work, so that one none the less tries to make the intervalsbetween servicing not all too short.

OBJECTS AND SUMMARY IF THE INVENTION

The present invention is based on the task of reducing operationalshutdowns which are necessary for maintenance work on dry-compressionpumps, but without increasing the risk of reduced service life or therisk of operational shutdowns due to malfunctioning dry-compressionvacuum pumps.

According to the present invention this task is solved by monitoringapplication-specific phenomena or their effects which lead to deviationsfrom the normal operating status and ensure, that after a predetermineddeviation quantity has been attained which makes apparent that in futureoperation of the vacuum pump might be adversely affected, measures canbe introduced to remove the causes thereof. The main advantage of thisproposal is that unnecessarily frequent shutdowns due to maintenancework can be avoided. Functioning of the pump is monitored constantly.Signs of imminent failures in the operation can be determined in duetime. Thus there then exists the possibility of extending the intervalsbetween servicing until the danger of operational failures is indicated.Since the time until such an indication occurs differs from applicationto application, the intervals between servicing can be optimally adaptedto each application.

BRIEF DESCRIPTION OF THE DRAWING

Further advantages and details of the present invention shall beexplained by referring to the drawing, wherein.

FIG. 1 is a partially broken away,side elevation view of a multi-stageclaws pump illustrating the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Shown in the drawing figure is a view of a multi-stage claws pump 1, asit is known from EU-A-365 695, for example. The last two stages 2 and 3are shown cut open at the level of one of the two shafts 4. The shaftsare driven with the aid of drive motor 5 via gear wheels which are notshown, and which also ensure synchronisation of shafts 4 and rotor pairs6, 7 in stages 2, 3. Rotor pairs 6, 7 are located in pump chambers 8, 9which are separated by side shields 11 (of which one is visible). Thelast stage 3 is formed by side shield 11 and an exhaust disc 12 which islocated in exhaust channel 13. The gases which are pumped through pump 1and which are ejected out of the last stage 3 enter through exhaustchannel 13 into a silencer 14 and from there they are released into theopen, possibly after having been cleaned.

Shafts 4 penetrate side shields 11 and exhaust disc 12. Labyrinth seals15 are provided at the level of the penetrations. Moreover, it is shownschematically that labyrinth seals 15 are equipped with a "purge seal"between shafts 4 and exhaust disc 12. For this, an inert gas, preferablynitrogen--is admitted into the respective space under the labyrinth seal15 via line 16 with gas rate monitor 17. A steady flow of inert gas ismaintained in the direction of pump chamber 9 via labyrinth seals 15 andthe slit between the corresponding rotor 7 and the related wall of thepump chamber. This prevents the entry of harmful substances from thegases pumped by claws pump 1 into the bearing and gear space 18 locatedunder the exhaust disc 12.

In order to be able to monitor operation of the claws pump 1 shown withrespect to deviations from normal operation due to the application,several alternatives are shown schematically which may be employedindividually, in multiples or also together. In all alternatives,typical operating parameters which may change due to process inducedphenomena are monitored. The signals detected by the measuringinstruments are supplied to a recording and display stage 21. It is thetask of this stage to initially indicate or provide a warning signal.This signal is provided at a point of time where the determinedapplication related phenomena have not already led to a malfunction inthe operation of the pump. The causes for later malfunctions can beremoved after this warning signal has been issued. If the warning signalis ignored, it will then be advantageous to generate an alarm signalwhen the quantity which deviates increases further. This will thensignal that it is now urgently required to remove the causes forpossible malfunctions; another possibility might be to shut down pump 1as soon as the alarm level is attained.

Monitoring the formation of layers within a stage of pump 1 may be basedon different indicators. For example, it is typical of a dry-compressionpump that the pressure ratio across the stages is, among otherthings--dependant on the return flow loss in the slit seals. Whendeposits form in these slits, play is reduced so that, initially, thecompression ratio increases, i.e., the characteristics of the pumpimprove. Thus the pressure ratio of a stage running at ultimate pressureor in the presence of a flow may be used as an indicator. The pressureratio is measured by pressure gauges 22, 23, the sensors 24, 25 of whichare located in the inlet or the exhaust, respectively, of the stageswhich are to be monitored (stage 3 in the case of the design exampleshown in the drawing figure). As the formation of deposits progresses,the pressure ratio (inlet pressure divided by exhaust pressure)increases. It will be required to empirically determine at whichpressure ratio levels the warning and the alarm signal will have to beissued. Also, monitoring of the improving ultimate pressure alonepermits conclusions to be drawn as to the formation of deposits in thepump chambers and on the rotors. However, for this it would have to benecessary to separate the pump from the vacuum chamber in which theprocess takes place.

Moreover, deposits and thus less play in the pump chambers lead tofluctuating motor currents. An increase in these fluctuations willtherefore point in the direction of increasing deposits and may beemployed for detection purposes. In the design example provided, thisalternative is indicated by sensor 26 which is related to drive motor 5and ammeter 27, the signal of which is supplied to recording and displaystage 21.

Deposits may also block exhaust channel 13 and/or the silencer 14. Ifeffects of this kind occur, then these can be detected relatively earlyby observing the power consumption of drive motor 5, i.e., by observingthe motor current with the aid of ammeter 27. There is the furtherpossibility of being able to detect these effects at an early stagethrough pressure measurements (pressure drop in the silencer with theaid of pressure sensors 31, 32 and measuring instruments 33, 34,observation of the exhaust pressure with the aid of measuring instrument23). In order to determine a beginning blockage of the silencer, itmight be sufficient to monitor the pressure in its inlet area (pressuresensor 31) alone. However, it will be of greater reliability todetermine the pressure drop, whereby two pressure sensors 31 and 32 arerequired.

Moreover, there exists the possibility of being able to capacitivelydetect deposits. For this purpose the electrodes of a capacitor 28 areintegrated into the wall of the pump chamber (pump chamber 9, in thedesign example shown). Deposits will cause the capacitance which ismeasured with the aid of measuring instrument 29 to change.

Besides this, there exists the possibility of being able to determinethe formation of deposits based on the inert gas rate which isdetermined by measuring instrument 17. The formation of deposits willreduce the amount of flowing inert gas per unit of time.

Finally, overloading the pump 1 with particles or liquids may also causethe motor current to increase or fluctuate. With respect to theseeffects, here too, monitoring of the motor current or its uniformity maybe utilised for early detection of malfunctions.

While this invention has been described in detail with reference to acertain preferred embodiment, it should be appreciated that the presentinvention is not limited to that precise embodiment. Rather, in view ofthe present disclosure which describes the best mode for practicing theinvention, many modifications and variations would present themselves tothose of skill in the art without departing from the scope and spirit ofthis invention, as defined in the following claims.

We claim:
 1. A method for monitoring deposition formation in a suctionchamber of a dry-sealing vacuum pump, said pump being connected to avacuum chamber in which a process occurs, said pump having at least onerotor and being driven by an electric motor, the method comprising thesteps of:integrating a capacitor into a wall of said suction chamber;monitoring a variance of capacitance of said capacitor; comparing saidvariance to a threshold value; and generating a warning signal if saidvariance exceeds said threshold value.
 2. A dry sealing vacuum pump,said pump being connected to a vacuum chamber in which a process occurs,said pump having a suction chamber, said pump having at least one rotorand being driven by an electric motor, said pump comprising:means forintegrating a capacitor into a wall of said suction chamber; monitoringmeans for monitoring a variance of capacitance of said capacitor;comparing means, connected to said monitoring means, for comparing saidvariance to a threshold value; and means, connected to said comparingmeans, for generating a warning signal if said variance exceeds saidthreshold value, therein indicating deposition formation in said suctionchamber.